Ceramic powders are typically formed into tiles and other pressed articles by depositing spray dried clay based powder into a cavity where it is pressed into a compressed shape. This is known in the art as dry forming. The powder is deposited into the cavity by many different methods including conveyor, single batch hoppers, and multiple batch hoppers.
The charging hopper, which is typically used, is a vibrating funnel-shaped delivery vessel that deposits spray dried granules into a cavity either directly, or onto a single-batch measured volume delivery tray. In each deposition, the spray dried granules in the hopper are shaken down to the bottom of the hopper adjacent to the hopper exit door. When deposition occurs, the hopper is opened and a pre-determined amount of material is delivered into feeder tray that delivers the powder into the cavity.
Continuous uniform powder deposition is difficult to maintain due to formation of powder bridges in the narrowing funnel-shaped delivery vessel which causes brief and irregular interruptions in the smooth flow of powder. This is acceptable in a system that deposits powder into a constrained volume such as a press cavity or batch delivery vessel since the powder in the cavity can be manipulated to disperse evenly.
Once the powder is deposited in a press cavity, it is pressed to form a shaped article. The pressed article is ejected from the cavity by a bottom plate and pushed over to a take-off table to go to a dryer and a decoration unit. After decoration, the article can be moved directly into the kiln for firing, or it can be pressed again to imprint a design into the surface of the article and then moved into the kiln for firing.
A problem that often occurs during pressing ceramic tiles is lamination. Lamination usually occurs due to trapped air during pressing in between the powder layers. These laminations cause problems during firing. Commonly, the trapped air can cause discontinuations in the ceramic matrix which weaken the final strength and can alter the shape of the fired article. One way these laminations are avoided is to press the article slowly with multiple repetitions with increasing force. Each time the article is pressed and released, more air is extracted out of the article and more compaction is achieved. This slow process eliminates the lamination in the final article, but it reduces throughput and decreases the efficiency of a production line.
Another problem with articles pressed in cavities is that when they are lifted or pushed out of the press cavity, the residual lateral pressure that has developed in the article is released when the article clears the wall of the mold. This lateral relaxation can easily lead to cracks on the edge of article especially for compositions with limited plasticity such as a composition containing a high amount of fly ash, for example up to 90%. Unlike a fly ash based composition, the composition of the spray dried clay-based powder has sufficient resilience to withstand these forces.
There are significant differences in behavior between clay powder and fly ash powder in this process. This invention enables fly ash based composition to be pressed into floor and wall tiles as well as other articles. The composition containing up to 90% fly ash is composed of mostly non-crystalline, non-plastic minerals with round Shapes and few angularities. This allows the fly ash particles to move freely past each other without catching or binding together, which lowers green strength. The green strength of a material is defined as its ability to hold itself together when handled or manipulated after high pressure press forming. Because of these smooth particle morphologies, fly ash articles which are pressed in preparation for firing have a much lower green strength. Standard spray-dried clay-based compositions can withstand approximately 4 to 8 kg/cm^2 of load (modulus of rupture test) before being dried. The composition containing up to 90% fly ash has a typical green strength of about 1.5 to 2 kg/cm^2. There are additives that can be used to increase the green strength of fly ash articles. These include water, clays, and superplasticizers such as those described in PCT WO 05/033040 A1 “Manufacture of Articles from Flyash”.
In a conventional press machine, as the machine shown in FIG. 1 and FIG. 2, the article 6 is pressed in a cavity 2 with press plates 1 and 3 and then lifted by a bottom plate level 4 to the surface of the work area where a pushing arm 5 moves it onto a conveyor to transport the article to the dryer and decoration unit or to the kiln for firing. Conventional spray dried clay-based compositions have sufficient green strength after pressing to withstand these manipulations. Articles made from compositions containing up to 90% fly ash, however, are typically too fragile, even after strengthening additives are used. This leads to an unacceptably high volume of damaged tiles, mostly due to the development of internal hairline cracks that will only be visible after firing.
As shown in FIG. 2, the conventional tile pressing process includes the following steps: at step 110, loading the cavity with powder; at step 120, pressing the powder with press plate, isostatic bottom plate level 4; at step 130, lifting off the top punch; step 140, raising up the pressed article 6; and at step 150, pushing the article 6 onto a conveyor 7 by a pushing arm 5 to the decoration or drying area.
CN2631726Y discloses a forming apparatus of ceramic tiles; the forming apparatus comprising a machine frame, a punching device uses cavity molding to press the powder article, an ejecting device, a guiding device, a governing plunger, a hydraulic control system for the ejecting device motion, and a feeding device toward a conveyer. The utility model can produce the ceramic tiles with different thicknesses.
For producing larger ceramic thin plate, CN 2631726Y discloses a transfer apparatus of ceramic tiles molding, the utility model provides a conveyor device in the ceramic tile molding equipment, which is composed of a press machine, an upper punch, conveyor or transporting belt, a belt wheel and a hopper. The feeding conveyor belt passes between the upper punch and the bottom die (cavity). The conveyor belt and the belt wheel combine to be a cyclic conveyor or system or a reciprocating conveyor system. And U.S. Pat. No. 5,521,132 discloses ceramic material (bar) made from raw coal fly ash of 90 wt. %, 92.5 wt. % and 95 wt. %, but it does not disclose how to press the powder to become a semi-dry bar.
U.S. Pat. No. 5,935,885 discloses a process for forming glass-ceramic tiles with fly ash from 60 wt. % to 100 wt. %. The fly ash is oxidized. The oxidized glass forming materials are vitrified to form a glass melt. This glass melt is then formed into tiles.
All of these disclosures are incorporated herein by reference.
In summary, one of the key challenges with using fly ash in standard ceramic tile manufacturing processes is that the pressed fly ash based composition has insufficient green strength to withstand the physical stresses involved when moving the tiles on to the conveyor belts to be transported for subsequent treatment.
The creation of a custom designed pressing and mould assembly was necessary to overcome the limitations of traditional ceramic tile press and mould assemblies that are not suitable for the manufacture of ceramic tiles using compositions containing 25% to 90% or even higher fly ash.
The currently available pressing and mould assemblies were tested and it was concluded that the following problems could not be overcome using commercially available methods.                A) Charging of the powder,        B) Lamination,        C) Cracking of edges upon ejection from the cavity,        D) Hairline cracks formed in article due to weaker than normal green strength,        E) Large percentage of breakage due to weaker than normal green strength,        F) Limited size of pressed article,        G) Reduced number of pressing strokes per minute.        
Subsequently an industrial scale custom build solution has been designed, engineered, and constructed that overcomes all of the problems associated with the above mentioned issues.